The invention relates to the field of light emitting diodes. More particularly, the invention relates to light emitting diode structures which provide improved current blocking and/or light extraction properties.
AlInGaP alloys have been used for making bright light emitting diodes (LEDs), wherein the light wavelength produced by an AlInGaP alloy LED is determined by the aluminum to gallium ratio of the alloy within the active region of the LED. The wavelength produced by an AlInGaP alloy LED is typically varied, from about 550 nanometers to about 680 nanometers.
A conventional AlInGaP LED typically contains a double heterostructure AlInGaP device, in which a first confining layer, such as an n-type AlInGaP, is formed on an n-type substrate, such as GaAs. An active layer or region of undoped. AlInGaP is then formed on the first layer, and a p-type AlInGaP confining layer is formed upon the active layer. Metalorganic vapor phase epitaxy (MOVPE) processes are typically used to grow the AlInGaP substrates for this double heterostructure device.
Various light emitting diodes have been disclosed in the prior art, which describe various LED structures, materials, and manufacturing processes. N. Hosoi, K. Fujii, A. Yamauchi, H. Gotoh, and Y. Sato, Semiconductor Light Emitting Devices, European Patent Application No. EP 0 702 414 A2 (filed Jan. 9, 1995) disclose various semiconductor light emitting device structures.
A. Dutta, Surface-Emission Type Light-Emitting Diode and Fabricating process Therefor U.S. Pat. No. 5,972,731 (Oct. 26, 1999), and U.S. Pat. No. 5,821,569 (Oct. 13, 1998), discloses xe2x80x9cAn n-type GaAs layer as a buffer layer, an n-type (Al0.7Ga0.3)0.5In0.5P layer, an active layer, a p-type (Al0.7Ga0.3)0.5In0.5P layer, a thin layer of AlxGa1xe2x88x92xAs layer (xxe2x89xa70.9), an Al0.7Ga0.3As layer as a current spreading layer and a high doped p-type GaAs cap layer are sequentially grown on an n-type GaAs layer of a substrate. As the active layer, an (AlxGa1xe2x88x92x)0.5In0.5P based bulk or multi-quantum well is employed. As the current spreading layer, an AlxGa1-xAs (xxe2x89xa70.7) is employed. The current spreading layer is a p-type III-IV compound semiconductor having wider band gap than a band gap of a material used for forming the active layer, and being established a lattice matching with the lower layer. After mesa etching up to the cladding layer, growth of selective oxide is performed at a part of the AlGaAs layer. By this, a block layer (selective oxide of AlGaAs) is formed. By this blocking layer, a light output power and a coupling efficiency are improvedxe2x80x9d.
K. Shimoyama, N. Hosoi, K. Fujii, A. Yamauchi, H. Gotoh and Y. Sato, Semiconductor Light-Emitting Devices, U.S. Pat. No. 5,811,839 (Sep. 22, 1998) disclose xe2x80x9ca semiconductor light-emitting device including a first clad layer comprising a first conductive type of AlGaAsP compound, a second clad layer that is located next to the first clad layer, comprises a first conductive type of AlGaInP compound and has a thickness of up to 0.5 xcexcm, an active layer that is located next to the second clad layer and comprises a first or second conductive type AlGaInP or GalnP, a third clad layer that is located next to the active layer, comprises a second conductive type of AlGaInP compound and has a thickness of up to 0.5 xcexcm, and a fourth clad layer that is located next to the third clad layer and comprises a second conductive type of AlGaAsP compound, and/or a light-extracting layer that comprises a second conductive type AlGaP or GaP and has a thickness of 1 xcexcm to 100 xcexcm.xe2x80x9d
H. Sugawara, M. Ishikawa, Y. Kokubun, Y. Nishikawa, S. Naritsuka, K. ltaya, G. Hatakoshi, and M. Suzuki, Semiconductor Light Emitting Device, U.S. Pat. No. 5,153,889 (Oct. 6, 1992) disclose xe2x80x9ca semiconductor light emitting device, comprising a semiconductor substrate, a double hetero structure portion formed on the front surface of the substrate and consisting of an InGaAlP active layer and lower and upper clad layers having the active layer sandwiched therebetween, a first electrode formed in a part of the surface of the double hetero structure portion, and a second electrode formed on the back surface of the substrate. A current diffusion layer formed of GaAIAs is interposed between the double hetero structure portion and the first electrode, said current diffusion layer having a thickness of 5 to 30 microns and a carrier concentration of 5xc3x971017 cmxe2x88x923 to 5xc3x971018 cmxe2x88x923.xe2x80x9d
J. Ming-Jiunn, B. Lee, and J. Tarn, Light Emitting Diode With Asymmetrical Energy Band Structure, U.S. Pat. No. 5,917,201 (Jun. 29, 1999) disclose a high bandgap material xe2x80x9cused as a cladding layer to confine the carrier overflow in a aluminum-gallium-indium-phosphide light emitting diode. The quantum efficiency is improved. The use of this high bandgap material as a window material also prevents current crowding. The efficiency can further be improved by using a Distributed Bragg Reflector in the structure to reflect light, and a buffer layer to reduce interface dislocation.xe2x80x9d
Y. Liu, Gallium Aluminum Arsenide Graded Index Waveguide, U.S. Pat. No. 4,152,044 (May 1, 1979) discloses a xe2x80x9cdouble heterostructure light emitting device has a graded index optical waveguide formed integrally therein. The integrally formed waveguide collects light from the heterojunction and directs the light in a distinct light pattern on one surface of the device. The rate of variation of the index gradient within the waveguide region determines the geometry of the light pattern. The light output pattern can be conveniently tailored to match the geometry of a wide variety of optical fiber dimensionsxe2x80x9d.
H. Abe, Semiconductor Light-Emitting Element with Light-Shielding Film, U.S. Pat. No. 5,192,985 (Mar. 9, 1993) discloses a semiconductor light-emitting element, which xe2x80x9cincludes a current pinching type semiconductor light-emitting element main body, which utilizes light extracted from a surface parallel to a light-emitting layer, and a light-shielding film, which is locally or entirely coated on a side surface of the semiconductor light-emitting element main body to be electrically insulated therefrom. A method of manufacturing a semiconductor light-emitting element, includes the steps of preparing a wafer by sequentially stacking and forming a current blocking layer, a first cladding layer, an active layer, a second cladding layer, and a first ohmic electrode on one surface of a substrate, and forming a second ohmic electrode on the other surface of the substrate, forming a resist film on the major surface of the wafer, forming a plurality of grooves reaching at least the first cladding layer at predetermined positions on the resist layer, coating an electrical insulating film on the resist film including the grooves, and coating a light-shielding layer on the electrical insulating film, removing the electrical insulating film, the light-shielding film, and the resist film so as to leave the electrical insulating film and the light-shielding film in portions of the grooves, and cutting the wafer at the portions of the grooves.xe2x80x9d
A. Cho, E. Schubert, L. Tu, and G. Zydzik, Light Emitting Diode, U.S. Pat. No. 5,226,053 (Jul. 6, 1993) disclose an LED in which: xe2x80x9can optical cavity of the LED, which includes an active layer (or region) and confining layers, is within a resonant Fabry-Perot cavity. The LED with the resonant cavity, hereinafter called Resonant Cavity LED or RCLED, has a higher spectral purity and higher light emission intensity relative to conventional LEDs. The Fabry-Perot cavity is formed by a highly reflective multilayer distributed Bragg reflector (DBR) mirror (RBxe2x89xa70.99) and a mirror with a low to moderate reflectivity (RTxcx9c0.25-0.99). The DBR mirror, placed in the RCLED structure between the substrate and the confining bottom layer, is used as a bottom mirror. Presence of the less reflective top mirror above the active region leads to an unexpected improvement in directional light emission characteristics. The use of a Fabry-Perot resonant cavity formed by these two mirrors results in optical spontaneous light emission from the active region, which is restricted to the modes of the cavity. While the bottom DBR mirror reduces absorption by the substrate of that light portion which is emitted toward the substrate, the two mirrors of the resonant cavity reduce the isotropic emission and improve the light emission characteristics in terms of a more directed (anisotropic) emission.xe2x80x9d
H. Kurikawa, Light Emitting Diode Including Active Layer Having First and Second Active Regions, U.S. Pat. No. 5,345,092 (Sep. 6, 1994) discloses a xe2x80x9clight emitting diode comprises a semiconductor substrate of compound semiconductor, an active layer provided above the semiconductor substrate and including first and second active regions, the first active region being spaced apart from the second active region thereby controlling diffusion of an injected minority carrier in a radial direction, the first active region substantially operating as a light emitting region, and a window for emitting light generated at the first active region.xe2x80x9d
F. Kish, F. Steranka, D. DeFevere, V. Robbins, and J. Uebbing, Wafer Bonding of Light Emitting Diode Layers, U.S. Pat. No. 5,502,316 (Mar. 26, 1996) disclose xe2x80x9cA method-of forming a light emitting diode (LED) includes providing a temporary growth substrate that is selected for compatibility with fabricating LED layers having desired mechanical characteristics. For example, lattice matching is an important consideration. LED layers are then grown on the temporary growth substrate. High crystal quality is thereby achieved, whereafter the temporary growth substrate can be removed. A second substrate is bonded to the LED layers utilizing a wafer bonding technique. The second substrate is selected for optical properties, rather than mechanical properties. Preferably, the second substrate is optically transparent and electrically conductive and the wafer bonding technique is carried out to achieve a low resistance interface between the second substrate and the LED layers. Wafer bonding can also be carried out to provide passivation or light-reflection or to define current flow.xe2x80x9d
Prior Current Spreading Structures
Efficient current spreading is critical to the performance of an LED. It is desirable to have applied current uniformly and quickly spread out over an LED device, before the current reaches the p-n junction. Since the aluminum content is normally high within the second confining layer, the electrical conductivity of the second confining layer is normally low, and it is generally difficult for the current to spread out quickly laterally across the second confining layer.
One common structure by which an applied current may be spread across the surface of an LED device is that of a relatively thick GaP or AlGaAs window layer, located above the double heterostructure (i.e. above the second confining layer), wherein current applied across the upper surface of the LED is spread as it extends downward through the thick window layer, before the current reaches the second confining layer and the p-n junction.
R. Fletcher, C. Kuo, T. Osentowski, and V. Robbins, Light-Emitting Diode with an Electrically Conductive Window, U.S. Pat. No. 5,008,718 (Apr. 16, 1991) disclose a light-emitting diode which has xe2x80x9ca semiconductor substrate underlying active p-n junction layers of AlGaInP for emitting light. A transparent window layer of semiconductor different from AIGaInP overlies the active layers and has a lower electrical resistivity than the active layers and a bandgap greater than the bandgap of the active layers, for minimizing current crowding from a metal electrical contact over the transparent window layer. The active layers may be epitaxially grown on a temporary GaAs substrate. A layer of lattice mismatched GaP is then grown on the active layers with the GaP having a bandgap greater than the bandgap of the active layers so that it is transparent to light emitted by the LED. The GaAs temporary substrate is then selectively etched away so that the GaP acts as a transparent substrate. A transparent window layer may be epitaxially grown over the active layers on the face previously adjacent to the GaAs substrate.xe2x80x9d
An alternate structure by which an applied current may be spread across the surface of an LED device is that of a conductive oxide layer, located above the double heterostructure, wherein current applied across the upper surface of the LED is spread laterally across the conductive oxide layer, and then extends downward toward the p-n junction. A contact layer is typically used in conjunction with a conductive oxide layer, to provide ohmic contact between the conductive oxide layer and the underlying LED layers.
M. Jou, C. Chang, B. Lee, and J. Lin, Surface Light Emitting Diode with Electrically Conductive Window Layer, U.S. Pat. No. 5,481,122 (Jan. 2, 1996) disclose a xe2x80x9csurface emitting AlGaInP LED having an ITO layer as a window layer to eliminate the current crowding effect, and an ohmic contact layer between its double hetero-structure of AlGaInP and the ITO layer, so that ITO can be utilized with the double hetero-structure of AlGaInP.xe2x80x9d
While it is generally desirable to have applied current uniformly and quickly spread out over the whole LED device, before the current reaches the p-n junction, LED structures typically have the top electrode located in a central region on the upper surface. Current flow which arrives at a region of the p-n junction directly below the electrode produces light which is shadowed by the electrode, resulting in inefficiency. In conventional LED structures, however, the current density is generally higher directly below the electrode. A large portion of the light generated within the p-n junction region under the electrode is then blocked by the electrode, as the generated light is transmitted upward. It would be advantageous to provide improved LED structures which have a greater power efficiency, by which applied current is more uniformly and quickly spread out over the xe2x80x9cunshadowedxe2x80x9d regions of the p-n junction, while current flow toward the p-n junction region directly under the electrode is reduced.
Prior Current Blocking Structures
Various structures have been disclosed to reduce current flow toward the p-n junction region under the top electrode.
H. Sugawara, M. Ishikawa, Y. Kokubun, Y. Nishikawa, and S. Naritsuka, Semiconductor Light Emitting Device, U.S. Pat. No. 5,048,035 (Sep. 10, 1991) disclose a xe2x80x9csemiconductor light emitting device, especially, a light emitting diode includes a compound semiconductor substrate of a first conductivity type, an InGaAIP layer formed on the substrate and having a light emitting region, a GaAIAs layer of a second conductivity type formed on the InGaAIP layer and having a larger band gap than that of the InGaAIP layer, and an electrode formed on a part of the GaAIAs layer. The light emitting diode emits light from a surface at the electrode side except for the electrode. A current from the electrode is widely spread by the GaAIAs layer to widely spread a light emitting region.xe2x80x9d While Sugawara et al. disclose a current blocking AlInGaP structure, the structure requires an extra epitaxial growth, to form the upper blocking region, and requires precise alignment. Such a structure is therefore. more complicated.
FIG. 1 shows a cross-sectional view of a light emitting diode 10 similar to the structure disclosed by Sugawara et al., having a substrate 14a established on a bottom electrode 12a, and a double heterostructure. 22a located on the substrate 14a, wherein the double heterostructure 22a comprises a first cladding layer 16a, an active layer 18a, and a second cladding layer 20a, and wherein a p-n junction 19a is typically established in the region between the active layer 18a and the first cladding layer 16a. A window layer 24a is then located on the second cladding layer 20a, and an upper blocking region 30 is located on the lower surface of a window layer 24a, on top of the double heterostructure 22a. An upper electrode 26a, having a contact layer 28a, is then connected to the upper surface of the window layer 24a. As seen in FIG. 1, when an applied power source 13a is connected between the lower electrode 12a and the upper electrode 26a, current 15a is directed toward the p-n junction 19a, and light 17a is produced in the active layer 18a. 
FIG. 2 shows a cross-sectional view of an alternate light emitting diode 32, having a substrate 14b formed on a bottom electrode 12b, and a double heterostructure 22b located on the substrate 14b, wherein the double heterostructure 22b comprises a first cladding layer 16b, an active layer 18b, and a second cladding layer 20b, and wherein a p-n junction 19b is established in the region between the active layer 18b and the first cladding layer 16b. A contact layer 28b is then located on the second cladding layer 20b, and an oxide layer 36a is formed on the upper surface of the contact layer 28b. An extended electrode 26b is located on the top of the LED structure 32, and extends through the oxide layer 36a and the contact layer 28b, to form a Schottky barrier 38a within the second cladding layer 20b. The alternate light emitting diode 32 provides current spreading 15b across the oxide layer 36a and associated contact layer 28b. As well, the oxide layer 36a and associated contact layer 28b may inherently absorb a portion of the light 17b produced within the light emitting diode 32. As well, while the extended conductive electrode 26b may provide current blocking, the conductive electrode structure 26b is inherently light absorbing, in that it blocks the transmission of light 17b which is produced underneath.
B. Lee, M. Jou, and J. Tarn, Light Emitting Diode Having Transparent Conductivity Oxide Formed on the Contact Layer, U.S. Pat. No. 5,789,768 (Aug. 4, 1998) disclose xe2x80x9ca substrate formed on a first electrode, a first cladding layer of a first conductivity type formed on the substrate, an active layer formed on the first cladding layer, a second cladding layer of a second conductivity type formed on the active layer, a window layer of the second conductivity type formed on the second cladding layer, wherein the electrical resistivity of the window layer is less than the electrical resistivity of the second cladding layer, a contact layer of the second conductivity type formed on the window layer for providing ohmic contact, a conductive transparent oxide layer formed on the contact layer, and a current blocking region formed in the LED. The current blocking region is approximately aligned with a second electrode, and can be the contact layer having a hollow portion therein, an insulating region formed on the contact layer, an ion implanted region in the contact layer and the window layer, or a diffused region in the contact layer and the window layer.xe2x80x9d While Lee et al. disclose an LED structure which includes current blocking and current spreading structures, the disclosed current blocking structure provides a shallow blocking depth, such that an applied current may readily flow laterally inward, toward the region under the blocking structure, as the current moves downfield though the window layer and the second cladding layer.
FIG. 3 shows a cross-sectional view of a light emitting diode 40 similar to the structure disclosed by Lee et al., having a substrate 14c formed on a bottom electrode 12c, and a double heterostructure 22c located on the substrate 14c, wherein the double heterostructure 22c comprises a first cladding layer 16c, an active layer 18c, and a second cladding layer 20c, and wherein a p-n junction 19c is established in the region between the active layer 18c and the first cladding layer 16c. A contact layer 28c is then located on the second cladding layer 20c, and an oxide layer 36b is formed on the upper surface of the contact layer 28c. An electrode 26c is located on the top of the oxide layer 36b. A hole is created within central region of the contact layer 28c, such that the oxide layer 36b extends through the contact layer 28c, and contacts the upper surface of the second cladding layer 20c, thereby forming a Schottky barrier 38b under the electrode 26c. 
While the Schottky barrier 38b may block a portion of the applied current flow 15c in the region under the under the electrode 26c, the blocking mechanism stops at the lower end of the contact layer 28c. Furthermore, while the oxide layer 36b and the contact layer 28c are typically very thin (e.g. such as a few hundred angstroms thick), the window layer 42 is typically much thicker (e.g. typically a few microns or thicker). Therefore, the Schottky barrier 38b typically provides a shallow blocking depth for the light emitting diode 40, and applied current 15c may readily flow laterally inward toward the region under the electrode 26c, as the current 15c moves downfield through the window layer 42 and the second confining layer 20. The current blocking efficiency of the Schottky barrier 38b is thus reduced, such that a significant portion of the light 17c which is produced by the structure 40 is produced by the central portion of the p-n junction 19c, and is either shadowed by the electrode 26c, or consequently produces a bright ring of emitted light 17c around the edge of the electrode 26c. 
As well, while the light emitting diode 40 shown in FIG. 3 includes an oxide layer 36a and an associated contact layer 28c to provide current spreading across the structure 40, the oxide layer 36a and associated contact layer 28c may inherently absorb a portion of the light 17c produced within the light emitting diode 32.
It would be advantageous to provide a light emitting diode structure which provides enhanced current blocking. It would also be advantageous to provide a light emitting diode structure which provides current spreading structure, while providing enhanced light transmission characteristics.
While the disclosed prior art light emitting diode structures provide current blocking structures in the region under the top electrode, they fail to provide a light emitting diode structure having a window layer, which provides current blocking beyond the top surface of the window layer. The development of such a light emitting diode structure would constitute a major technological advance.
As well, while some of the conventional prior art light emitting diode structures provide basic current spreading structures, they fail to provide a light emitting diode structure that includes a current spreading structure, while providing enhanced light transmission. The development of such a light emitting diode structure would constitute a major technological advance.
Structures for light emitting diodes are disclosed, which include improved current blocking and light extraction structures. The diodes typically include a substrate formed on a first electrode, a first confining layer of a first conductivity type formed on the substrate, an active region formed on the first confining layer, a second confining layer of a second conductivity type formed on the active region, and a window layer of the second conductivity type formed on the second confining layer. A contact layer of the second conductivity type is formed on the window layer, a conductive oxide layer is formed on the contact layer, and a second electrode is formed on the conductive oxide layer.
The conductive oxide layer typically includes a central portion located below the second electrode, which-extends into the LED structure, preferably beyond the contact layer and into the window layer, or even beyond the window layer, such as into the second confining layer, or even beyond the second confining layer, into the active layer. The dimension of the second electrode is preferably smaller than that of the central extending portion of the conductive oxide layer. In alternate embodiments of the improved light emitting diode, the central extending portion may be a separate conductive region from the conductive oxide layer.
A resistive or reverse-biased pattern or region is preferably provided below the active layer, to provide enhanced current blocking, wherein the pattern is located in the substrate, or in the first confining layer, and is approximately aligned below the second electrode. The dimension of the resistive or reverse-biased pattern is preferably similar to or larger than the current blocking dimension, which is preferably larger than the second electrode.
The improved light emitting diodes preferably include one or more holes, which are defined in the conductive oxide layer, or within both the conductive oxide layer and the contact layer, to promote the transmission of light from the upper surface of the light emitting diode.
A Distributed Bragg Reflector is also preferably provided between the substrate and the first confining layer, to reduce light absorption within the substrate, and to promote efficient light extraction from the top of the LED structure.